1. Field of the Invention
This invention relates to microelectromechanical devices, and more particularly, to a microelectromechanical device in which includes one or more contact structures interposed between a pair of electrodes.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Microelectromechanical devices, or devices made using microelectromechanical systems (MEMS) technology, are of interest in part because of their potential for allowing integration of high-quality devices with circuits formed using integrated circuit (IC) technology. For example, MEMS switches may exhibit lower losses and a higher ratio of off-impedance to on-impedance as compared to transistor switches formed from conventional IC technology. However, a persistent problem with implementation of MEMS switches has been the high voltage required (often about 40V or higher) to actuate the switches, as compared to typical IC operating voltages (about 5V or lower).
These relatively high actuation voltages of MEMS switches are caused at least in part by a tradeoff between the closing and opening effectiveness of a given switch design. For example, approaches to lowering the actuation voltage of switches have included reducing the stiffness of the switch beam and/or reducing the gap between the beam and the conductive pad. Unfortunately, these design changes typically result in making the switch more difficult to open. MEMS switch designs generally use an applied voltage to close the switch, and rely on the spring force in the beam to open the switch when the applied voltage is removed. In opening the switch, the spring force, or restoring force, of the beam must typically counteract what is often called xe2x80x9cstiction.xe2x80x9d Stiction refers to various forces tending to make two surfaces stick together such as van der Waals forces, surface tension caused by moisture between the surfaces, and/or bonding between the surfaces (e.g., through metallic diffusion). In general, modifications to a switch which act to lower the closing voltage also tend to make the switch harder to open, such that efforts to form a switch with a lowered closing voltage can result in a switch which may not open reliably (or at all).
Another limitation typically found in MEMS devices is the presence of residual stresses contained within the switch beam. In particular, the residual stresses within a beam may cause the beam to curl either away from contact structures or toward contact structures without the activation of actuation voltages. In the event that the beam curls down and closes a contact prematurely, the switch may become inoperable because significant electrostatic repulsion between the gate and the beam cannot be established. In this manner, the switch may not be opened by removing an applied voltage as described above. In other cases, MEMS devices may be designed such that the functionality of the devices is dependent on the presence of residual stresses within the beam. For example, a device may be designed to have compressive stresses within the beam in order to curl one portion of the beam in one direction as another portion of the beam curls in the opposite direction. Actuation voltages may then be used to oscillate the beam between its original state and the mirror image of the original state. Such a device, however, may be difficult to consistently fabricate since residual stresses within the beam may be dependent on the properties of the beam materials and such properties may change as variables of the fabrication process change. In addition, the range of materials used within such a MEMS device may be limited.
It would therefore be desirable to develop a MEMS device which relaxes the constraints imposed by the above-described tradeoff between opening and closing effectiveness and the presence of residual stresses within the device.
The problems outlined above may be in large part addressed by a microelectromechanical device which includes a contact structure interposed between a pair of electrodes arranged beneath a beam. In some embodiments, the device may include additional contact structures interposed between the pair of electrodes. For example, the device may include at least three contact structures between the pair of electrodes. Preferably, the additional contact structures may be spaced along and under a length of the beam adjacent to the contact structure. In some embodiments, the beam may be suspended above the pair of electrodes by a support structure affixed to a first end of the beam. Such a device may further include an additional support structure affixed to a second end of the beam. In some cases, the device may be adapted to pass a signal from the first end to the second end of the beam. In addition or alternatively, the device may be adapted to pass the signal between one and/or both ends of the beam and one or more of the contact structures.
As stated above, a microelectromechanical device is provided which includes a contact structure interposed between a pair of electrodes arranged beneath a beam. In some embodiments, the beam may be suspended by support structures affixed to first and second ends of the beam. Alternatively, the beam may be solely supported at one end of the beam. In either embodiment, the device may be adapted to pass a signal between one and/or both ends of the beam and the contact structure. In addition or alternatively, the device may be adapted to pass a signal from the first end to the second end of the beam. In particular, the device may be adapted to pass the signal through the beam upon bringing the beam in contact with the contact structure. In an alternative embodiment, the device may be adapted to pass the signal through the beam without bringing the beam in contact with the contact structure interposed between the pair of electrodes. In such an embodiment, the beam may include a contiguous layer of conductive material. Alternatively, the beam may include an insulating element interposed between the first and second ends of the beam.
In some embodiments, the device may include one or more additional contact structures interposed between the pair of electrodes. In such an embodiment, the upper surface of the contact structure may be above or below the upper surfaces of the additional contact structures. In some cases, the contact structure may include a raised section arranged upon its upper surface. Alternatively, the upper surface of the contact structure may be approximately level with upper surfaces of the additional contact structures. In such an embodiment, the beam may include a recessed portion above the contact structure. In some cases, one or more of the additional contact structures and contact structure may include multiple sections spaced apart from each other and arranged along the width of the beam.
In some cases, the device may be adapted to bring the beam in contact with one or more of the contact structures. For example, the beam may include residual forces adapted to bring the beam in contact with one or more of the contact structures. Such residual forces may be further adapted to curl the beam away from one or more of the contact structures distinct from the contact structures in contact with the beam. In an alternative embodiment, the device may be adapted to bring the beam in contact with one or more of the contact structures upon an application of one or more closing voltages to at least one of the pair of electrodes. More specifically, the beam may be brought in contact with one or more of the contact structures upon applying a closing voltage to one of the pair of electrodes. In an alternative embodiment, the beam may be brought in contact with one or more of the contact structures upon applying a closing voltage to both of the pair of electrodes. In yet another embodiment, the device may be adapted to bring the beam in contact with one or more of the contact structures by other forms of actuation on the controlling elements (e.g., gate electrodes) of the device. For example, the device may be adapted to bring the beam in contact with one or more of the contact structures upon magnetic, piezoelectric, or thermal actuation.
In either embodiment, the device may be adapted to pull the beam away from the one or more contact structures in contact with the beam. For example, an application of an actuation voltage to one of the pair of electrodes may pull the beam away from the one or more contact structures in contact with the beam. In some embodiments, such an actuation voltage may be adapted to bring the beam in contact with one or more contact structures, which are not previously in contact with the beam. Alternatively, the application of such an actuation voltage may not bring the beam in contact with any additional contact structures. In an embodiment in which a closing voltage is applied to one of the pair of electrodes to bring the beam in contact with one or more contact structures, the device may be adapted to pull the beam away from one or more of the contact structures upon a release of the closing voltage during or after an application of an actuation voltage to the other of the pair of electrodes. Alternatively, such a device may be adapted to pull the beam away from one or more of the contact structures upon an increase of an actuation voltage applied to the other of the pair of electrodes during or after a release of the closing voltage.
In an embodiment, the device may include at least three contact structures interposed between the pair of electrodes. In such an embodiment, the contact structures and pair of electrodes may be laterally spaced along and under a length of a beam. As stated above, the device may include a support structure attached to a first end of the beam. In some cases, the device may include an additional support structure attached to a second end of the beam. In either embodiment, the device may include a first contact structure interposed between the pair of electrodes and a second contact structure interposed between the first contact structure and one of the pair of electrodes. Such a device may further include a third contact structure interposed between the first contact structure and the other of the pair of electrodes. In some cases, the first contact structure may be arranged under the center point of the beam. Alternatively, the first contact structure may be arranged closer to the one of said pair of electrodes than to the other of said pair of electrodes. In some embodiments, the device may further include one or more contact structures interposed between said first and second contact structures. In addition or alternatively, the device may include one or more contact structures interposed between said second contact structure and the one of the pair of electrodes.
A method for fabricating the microelectromechanical device as described above is also contemplated herein. In particular, the method may include patterning an array of contact structures between a pair of electrodes. In some embodiments, patterning the contact structures may include patterning base structures of the contact structures and then patterning a raised section upon the upper portion at least one of the base structures. In addition or alternatively, the method may include forming support structures laterally adjacent the sides of the pair of electrodes facing away from the array of contact structures. The method may further include forming a beam spaced above the electrodes and contact structures such that the beam is supported at its respective lateral ends. In some cases, forming the beam may include forming a sacrificial layer upon the electrodes, the contact structures, and exposed portions of the substrate. A beam layer may be deposited upon said sacrificial layer and the sacrificial layer may be subsequently removed. Forming the sacrificial layer may include, for example, depositing the sacrificial layer upon the pair of electrodes, the contact structures, and exposed portions of the substrate and etching recesses within the sacrificial layer. In some cases, etching the sacrificial layer may include etching recesses above at least one of the contact structures. In addition or alternatively, etching the sacrificial layer may include etching recesses laterally adjacent to sides of the pair of electrodes facing away from the array of contact structures.
There may be several advantages to forming a device that includes a contact structure interposed between a pair of electrodes arranged beneath a beam. For example, either of the pair of electrodes may be used to bring the beam in contact with the contact structure, allowing greater design flexibility in the device. Alternatively, both of the pair of electrodes may be used to bring the beam in contact with the contact structure. In such an embodiment, the device may operate at lower actuating voltages, thereby making implementation with integrated circuits more feasible. Moreover, further advantages may be realized in embodiments in which additional contact structures are interposed between the pair of electrodes. For example, a device with a plurality of contact structures interposed between a pair of electrodes may overcome the opening difficulties associated with surface tension issues, such as stiction. As such, a more flexible beam may be employed within the device. Consequently, the device may operate at lower actuating voltages. In addition, the functionality of the device as described herein is not restricted by residual stresses contained within the device since a repulsive electrostatic force between the gate and beam is not required to exist in order to deflect the cantilever from the conductive pad. In other words, the device as described herein may deflect a beam that has come in contact with a contact structure without the influence of a repulsive electrostatic force.